1. Field of the Invention
The present invention generally relates to a portable device for regulated production of heat by catalytic reaction, and more particularly to a portable heat generating device in which heat is uniformly generated across the surface of a thin sheet-shaped, elastomeric structure.
2. Description of the Prior Art
A variety of portable chemical heat generating devices are known which can be incorporated into, for example, outerwear, garments and blankets.
A first type of device is taught in U.S. Pat. No. 4,516,564 and U.S. Pat. No. 4,756,299. This first type of device includes a powdered, exothermic material, such as oxidizable metal, which is maintained in a sheet-like form and covered with a porous, air permeable sheet. The amount of air permeating the sheet is regulated to control the reaction rate of the exothermic materials, thereby controlling the amount of generated heat.
A second device is taught in U.S. Pat. No. 5,425,975. In this second device, exothermic material is dispersed in and supported by a sheet-like substrate made up of a plurality of irregularly arranged fibers having a multiplicity of gaps there between which facilitate air flow to the exothermic material. The sheet-like substrate is held in a bag having air-permeation holes. As with the first type of device, the amount of air entering the sheet-like substrate passing through the gaps is controlled such that the exothermic material generates a desired amount of heat. A third device is taught in U.S. Pat. No. 5,125,392. In this device, exothermic material is held within a multitude of holes formed in a thermogenic material mat located between a pair of panels. Air is supplied to the exothermic material by a pump through a first plurality of air passages, and exhaust gases exit though a second plurality of air passages. The amount of heat generated by the exothermic material is controlled by controlling the air flow through the pump.
A problem associated with the above-mentioned first, second and third known device types is that the exothermic material is depleted after a period of use, thereby terminating the heat generating process. When the exothermic material is depleted, it is necessary to either dispose of some or all of the heating device, or to perform a cumbersome and time consuming process of replacing or regenerating the exothermic material. These characteristics make such devices impractical for multi-day travel on foot in isolated geographic locations where weight, convenience and refuse considerations are important.
Another problem associated with the above-mentioned first and second device types is that heat production is turned on and off relatively slow because it is regulated by means of natural diffusion of air through permeable membranes of large surface area. Further, if these devices are used for warming parts of the body other than the extremities, turning these devices off requires physical removal of the devices from the body and storage in an air tight compartment. Because these heating devices are usually worn under a passive outer garment in these instances, they are not well suited for heat-on-demand applications where it is impractical or inconvenient to remove the outer layers of clothing.
The above-mentioned first and second device types also suffer from the inability to provide a wide range of thermal power output. In order to insure that the devices do not produce unsafe temperature, their maximum thermal power production, even under the best conditions, must by necessity be fixed and limited to a relatively low value. Thus, the potentially high power production of the above chemical heaters are never really made available to the user when the environmental conditions might justify it.
A fourth portable heat generating device is taught in U.S. Pat. No. 4,685,442. This portable heating device generates heat in a heat exchanger with is mounted at a location remote from the desired point of application of the heat. A circulating heat transfer fluid is pumped through the heat exchanger and then delivered to a remote location to perform the warming function. However, because of heat loss from the heat transfer fluid as it travels to the desired point and the intrinsic nature of heat exchange processes in general, the energy efficiency of this device is relatively poor. Furthermore, the device is relatively heavy because, in addition to the fuel required to provide the heat energy, the heat transfer liquid is required to transport the heat to the desired point. Another shortcoming of the fourth device is that the heat transfer fluid retains heat for a significant period of time after extinguishing the heat source because of the high heat capacity of liquids (i.e. as compared to gasses), thus preventing rapid regulation of the heat supply.
A fifth portable heat generating device is taught in U.S. Pat. No. 2,764,969. This device utilizes the flameless combustion principle and methanol based fuel-air mixture, however, it makes no provision for the safe handling of any unburned fuel or products of incomplete combustion. Any catalytic portable heat generating device that is used in close personal contact with the human body or in confined spaces such as a tent, vehicle or small room would be deemed impractical and unsafe if products of incomplete combustion or volatile organic compounds (VOC's) were released to the ambient during the heating process. In addition, the above fifth mentioned device type suggests using 7/8 inch diameter tubing within the garment which is substantially intrusive with regard to use in outerwear. Furthermore, the inner tubing material is made of rigid and semi-rigid metal structures that further reduce the ability to be worn comfortably. Also, the method of combustion used in the fifth mentioned device type generally requires much higher temperatures at the reaction surface (the surface in direct contact with the catalytic material) than the present invention since heat is transferred from the reaction surface to the outer surface indirectly and over a relatively large gap. Furthermore, to avoid dangerous surface temperatures, it would appear that the outer tube diameter (i.e. 7/8 inch) may not be reduced significantly below the diameter specified. In any case, significant reduction in the tubing diameter would likewise limit the total power that can be radiated at safe surface temperatures (e.g. less than 120.degree. F.) because of the small surface area per unit length of the cylindrical geometry, as compared to a sheet like geometry.
Yet another problem with the above mentioned fifth device type is that no provision is made to avoid problems that may occur during portions of the operation cycle when condensation of water vapor (i.e. a combustion by-product) within the tubing may cause self-extinguishment of the combustion process or prevent re-start after shutting off the apparatus. It has been found that a fast heat-up of a catalytic heat element while the channel wall is still cool or a fast cool down of the envelope containing the heat element or a rapid change in operating conditions (e.g. flow rate, fuel/air ratio, ambient temperature, etc.) may cause condensation within the channels. Furthermore, for many applications it is desirable to operate a catalytic heater in the following manner:
(a) Relatively low surface temperature of catalytic heat element; to allow the use of elastomeric plastics as the primary component in construction of the heat sheet. PA1 (b) Low fuel-air flow rates; to minimize air pump size, weight and power requirements. PA1 (c) Relatively high fuel/air ratio; to allow high power levels and system efficiencies when operating at low flow rates.
Each operating constraint listed in items (a) through (c) can exacerbate potential condensation effects and therefore may be problematic unless some remedy is employed. In addition, none of the prior art attempts to optimize all three of the above items (a) through (c).
Prior art catalytic heaters, as taught for instance in U.S. Pat. No. 4,140,247, U.S. Pat. No. 3,191,659, U.S. Pat. No. 3,198,240, and U.S. Pat. No. 5,282,740, typically operate at high reaction surface temperatures with relatively high gas flow rates and generally release their exhaust products immediately to the atmosphere, thus avoiding concern about water condensation interfering with heater operation. In U.S. Pat. No. 4,662,352, the fuel/air ratio is kept low, between 1% to 3% fuel-to-air ratio (by volume), thus avoiding problems with water condensation, as well as, avoiding significant spatial asymmetries in the combustion process (i.e. combustion occurring largely in the vicinity of where the fuel-air stream first contacts the catalytic material). However, this approach would not be efficient if applied to a personal heat device where significant power levels at low power densities and low flow rates are desired.
Yet another problem with prior art catalytic heaters, as inferred in item (a) above, is that the relatively high reaction temperatures require the use of metallic structures and other rigid materials in the construction of the heater, preventing implementation of a substantially all synthetic polymer construction that would allow the device to achieve the optimum tactile, flexible and pliant character required for comfortable and unobtrusive inclusion into outerwear.
All of these shortcomings, as well as, others associated with prior art chemical heat generating devices, limits their applications or area of use. The present invention provides a novel approach to overcome these difficulties and appreciably increase marketability for use in, for example, outerwear, garments, blankets and sleeping bags, and the like.